The basic component elements of a proximity sensor include a detection coil assembly having a coil and a core, an oscillation circuit unit, an output circuit unit and an outer shell case for accommodating these elements. In order to meet the users' demand for a wide range of product specifications in versatile way, the makers are required to reduce the cost by concentrating the basic component parts as far as possible.
In promoting the cost reduction by concentration of the component parts of a proximity sensor, various problems unique to the proximity sensor are posed from the viewpoints of both magnetic and electric circuits.
With regard to the magnetic circuit, for example, the cost reduction by the concentration of component parts poses the following problems in respect of the characteristics and structure of the detection coil assembly.
The proximity sensor of induction type has a detection coil assembly responsive to the approach of a metal. This detection coil assembly includes a coil and a ferrite core. The approach of a metal is detected utilizing the change in the characteristic of the detection coil assembly. The body case (outer shell case) of the proximity sensor is formed of any of various materials including brass, stainless steel and resin due to the variety of the operating environments. The detection performance of the proximity sensor changes also with the outer diameter of the detection coil assembly. For this reason, many variations of the detection coil assembly of cylindrical type having different outer diameters are prepared including M8 (about 8 mm in outer diameter), M12 (about 12 mm in outer diameter) and M30 (about 30 mm in outer diameter).
The detection characteristic of the detection coil assembly of the proximity sensor of induction type is affected to a large measure by the variations in the material and the dimensions of the body case (outer shell case), the plating thickness of the body case and the relative assembly positions of the detection coil assembly and the body case. As a result, the various problems are encountered as described below.    (1) To provide products of different materials, shapes and sizes of the body case, the detection coil assembly and the oscillation circuit are required to be designed both individually and as need arises. This leads to an increased number of design steps and an increased cost on the one hand and an increased parts cost due to the difficulty of parts concentration on the other. The production line is also required to be designed individually, thereby requiring a plurality of lines and steps.    (2) To eliminate the variations of the detection distance, the relative positions of the detection coil assembly and the body case are required to be accurate, the quality such as the dimensional accuracy of the body case must be improved, and an ingenious method of adjusting the detection distance is required, thereby leading to a higher parts cost and an increased number of production steps and production cost.
The above-mentioned problem points of the proximity sensor of induction type will be discussed in more detail below with reference to several specific examples of the detection coil assembly.
Detection Coil Assembly According to Prior Art 1:
FIG. 23 shows a structure of the detection coil assembly of shielded type (which is defined as the one capable of being buried in a metal on which it is mounted) having a metal body case.
In this drawing, numeral 101 designates a cylindrical body case (outer shell case) of a metal such as brass or stainless steel, numeral 101a a male screw formed on the outer peripheral surface of the body case, numeral 102 a ferrite core, numeral 103 a coil making up a detection coil, and numeral 104 an insulating internal case in the shape of a bottomed cylinder for accommodating the core 102 and the coil 103.
In the detection coil assembly of this structure, the detection characteristic of the detection coil assembly varies to a large measure depending on the material of the body case 101 located on the outer periphery of the coil 103 and the core 102. This is probably because the body case 101 is also located in the magnetic field formed by the coil 103.
The detection characteristic of the detection coil assembly according to the prior art 1 is shown in the graph of FIG. 24. This graph is plotted based on the measurement data of the detection coil assembly of M8 shielded type. The abscissa represents the detection distance (mm) and the ordinate the conductance g (μS) of the detection coil. The conductance of the detection coil increases with the approach of a metal object. The proximity sensor operates on the principle that a detection signal is generated by utilizing the change in the conductance g of the detection coil. As apparent from this graph, variations of the material of the body case 101 including resin (nonmetal), brass and stainless steel are seen to greatly change the conductance characteristic (i.e. detection characteristic) correspondingly. From this, it will be understood that the circuit constants (the constants and the threshold value of the oscillation amplitude of the oscillation circuit, etc.) of the detection circuit are required to be changed in accordance with the material of the body case 101, thereby making it difficult to concentrate the parts.
FIG. 25 shows a structure of a detection coil assembly of unshielded type having a metal body case (defined as the one not suitable for use in the form buried in a mounting metal and having a longer detection distance than the shielded type).
In this drawing, numeral 107 designates a cylindrical body case (outer shell case) of a metal such as brass or stainless steel, numeral 107a a male screw formed on the outer peripheral surface of the body case, numeral 102 a ferrite core, numeral 103 a coil making up a detection coil, and numeral 108 a coil case in the shape of a bottomed cylinder made of resin for accommodating the coil 103 and the core 102. As apparent from the drawing, the resin coil case 108 has a structure protruded forward of the metal body case 107.
In the detection coil assembly of this structure, the difference in material of the body case 107 located rearward of the coil 103 and the core 102 greatly changes the detection characteristic of the detection coil assembly.
The detection characteristic of the detection coil assembly according to the prior art 2 is shown in the graph of FIG. 26. This graph is plotted based on the measurement data of the detection coil assembly of M8 unshielded type. The abscissa represents the detection distance (mm), and the ordinate the conductance g (μS) of the detection coil. The approach of a metal object increases the conductance g of the detection coil. The proximity sensor operates on the principle that a detection signal is generated utilizing the change of the conductance g of the detection coil. As apparent from this graph, the different materials including resin (nonmetal), brass and stainless steel of the body case 107 are known to result in correspondingly quite different conductance characteristics (i.e. different detection characteristics). From this, it will be understood that the circuit constants (the constants, the threshold value of the oscillation amplitude of the oscillation circuit, etc.) of the detection circuit are required to be changed in accordance with the material of the body case 107, thereby making it difficult to concentrate the parts.
Next, from the viewpoint as an electrical circuit, the reduction in cost by concentration of component parts encounters the following problem due to the configuration of the oscillation circuit and the output circuit as well as the characteristic and structure of the detection coil assembly.
As described already, the oscillation conditions of the oscillation circuit including the coil of the induction-type proximity sensor change with the approach of a metal object. By detecting the change of the oscillation conditions, therefore, a metal object is detected. In the proximity sensor of induction type, the detection performance also changes with the outer diameter of the coil of the detection coil assembly. Therefore, variations of proximity sensor having different outer diameters are prepared. As a result, the oscillation circuit constant is determined for each coil outer diameter used or the detection distance adjusted.
On the other hand, assume a sensor application at the production site, etc. in which the presence or absence of a metal work is detected and various actuators are controlled by this detection signal. The configuration of the power line and the signal line connected to the proximity sensor is divided into DC three-line type, DC two-line type and AC two-line type to suit various situations. Also, the output status is classified into NPN or PNP type, voltage output or current output type, detection-related drive type or nondetection-related drive type, etc. In commercializing a proximity sensor, therefore, many circuit variations are prepared in accordance with the power form and output status of the respective proximity sensors. In keeping with the demand for a compact proximity sensor, the latest trend is toward concentration of the oscillation circuit, the power supply, the output circuit, etc. into a one-chip IC.
With this background, in designing a proximity sensor, an attempt to renew a given function (such as an increased detection distance, a reduced circuit current consumption, a lower voltage for driving the power supply driven, etc.) is accompanied by the need of remaking all the circuits of the proximity sensor into a one-chip IC. In addition, to meet the great commodity variations, the constants of the parts built around the IC and the coil constant require redesigning. As a result, the following various problems present themselves.    (1) The development cost of a new commodity is tremendously high.    (2) The commodities meeting the operating conditions of the user of the proximity sensor cannot be provided immediately.    (3) The vast commodity variations and the resulting multi-item scant production, which increases the production cost, the management cost and the parts cost, lead to a higher overall production cost.    (4) The replacement design and the acquisition work in case of suspension of the parts production are vast in amount.    (5) In the case where a quality problem shared by various products arises, the requirements for all the commodities cannot be quickly met.
The above-mentioned problem points of the proximity sensor of induction type will be explained in more detail with reference to several specific examples of the oscillation and output circuits.
Proximity Sensor Circuit According to Prior Art 1:
FIG. 27 shows a circuit configuration of a proximity sensor of DC three-line type. In this drawing, numeral 200 designates a metal object (such as a metal work), numeral 201 a custom IC, numeral 202 an oscillation circuit, numeral 203 an integration circuit, numeral 204 a discrimination circuit, numeral 205 a logic circuit, numeral 206 an output control circuit, numeral 207 a constant voltage circuit, numeral 208 a power reset circuit, numeral 209 a shorting protection circuit, numeral 210 a display circuit, numeral 211 a detection coil, numeral 212 a capacitor making up a resonance circuit, numeral 213 a regulation circuit, numeral 214 a capacitor making up an integration circuit, numeral 215 an output transistor, numeral 216 a light-emitting element, numeral 217 a first power terminal, numeral 218 a second power terminal, and numeral 219 a signal output terminal.
Proximity Sensor Circuit According to Prior Art 2:
FIG. 28 shows a circuit configuration of a proximity sensor of DC two-line type. In this drawing, numeral 220 designates a metal object (such as a metal work), numeral 221 a custom IC, numeral 222 an oscillation circuit, numeral 223 an integration circuit, numeral 224 a discrimination circuit, numeral 225 a logic circuit, numeral 226 an output control circuit, numeral 227 a constant voltage circuit, numeral 228 a power reset circuit, numeral 229 a shorting protection circuit, numeral 230 a display circuit, numeral 231 a detection coil, numeral 232 a capacitor making up a resonance circuit, numeral 233 a regulation circuit, numeral 234 a capacitor making up an integration circuit, numeral 235 an output transistor, numerals 236, 237 light-emitting elements, numeral 238 a first power terminal, and numeral 239 a second power terminal.
The prior art 1 and the prior art 2 described above share substantially all the circuit elements except for the difference in power system. As a representative of the two, therefore, the configuration and operation of the circuit elements of the prior art 2 shown in FIG. 28 will be explained below.
The detection coil 231 is a solid copper wire or twisted Litz wire wound by an appropriate number of turns. The resonance capacitor 232 is connected in parallel to the detection coil 231 and makes up a LC parallel resonance circuit, which is connected to the oscillation circuit 222. The custom IC 221 is an IC (integrated circuit) with the main circuits of the proximity sensor built in one chip. This custom IC 221 has built therein the oscillation circuit 222, the integration circuit 223, the discrimination circuit 224, the logic circuit 225, the output control circuit 226, the constant voltage circuit 227, the power reset circuit 228, the shorting protection circuit 229 and the display circuit 230. The regulation circuit 223 includes a plurality of resistors combined. By changing the resistance value of a part of the resistors by replacement or by laser trimming or otherwise, the gain of the oscillation circuit 222 is changed, thereby making it possible to adjust the detection sensitivity of the proximity sensor. The integration capacitor 234 is combined with the integration circuit 223 to make up a CR integration circuit. The output transistor 235 generates a large drive current based on the control signal (CONT) output from the custom IC 221. Numeral 236 designates an operation indication lamp for indicating the output operating conditions of the proximity sensor. Numeral 237 designates a setting indication lamp indicating a set position which assures positive detection against any variations of the detection distance due to the operating environment. Numerals 238, 239 designate power supply terminals led out of the proximity sensor through a cord and a connector. This case involves a two-line proximity sensor, and therefore the first power terminal 238 doubles as an output terminal.
FIG. 29 shows an example of a specific circuit configuration of the oscillation circuit, the integration circuit and the discrimination circuit according to the prior art 2 shown in FIG. 28. FIG. 30 is an operation time chart for the circuits shown in FIG. 29.
As shown in these diagrams, the oscillation voltage ((b) of FIG. 30) of the oscillation circuit 222 is smoothed by the integration circuit 223. The output ((c) of FIG. 30) thus smoothed is binarized by being compared with the reference voltages C and D of the discrimination circuit 224, thereby producing the detection signals E and F in binary form ((d), (e) of FIG. 30).
These detection signals E, F are sent to the output control circuit 226 through the logic circuit 225, and controls the output transistor 235 thereby to turn on/off the output 238 of the proximity sensor ((f) of FIG. 30), while at the same time turning on/off the operation indication lamp 236 and the setting indication lamp 237 ((g), (h) of FIG. 30).
The oscillation circuit 222 has such a characteristic that the oscillation amplitude thereof is changed substantially linearly in accordance with the distance by which a metal object approaches. As long as the metal object 230 has not approached, the oscillation amplitude A ((b) of FIG. 30) is sufficiently large, and the binarized detection signals E, F are off ((d), (e) of FIG. 30). With the approach of the metal object 230, the oscillation amplitude A and the integration output B are reduced gradually with the distance covered by the approach. In the case where the integration output B is reduced to or below the reference voltage C, the detection signal E is turned on. In the case where the integration output B is reduced to or below the reference voltage D, on the other hand, the detection signal F is turned on. The detection signals E, F are logically processed in the logic circuit 225 shown in FIG. 28 and sent to the output control circuit 226. After that, the output control circuit 226 operates to drive the output transistor 235.
The constant voltage circuit 227 is supplied with power from the power terminals 238, 239 and thus generates a constant voltage output while at the same time driving each internal circuit. On the other hand, a minimum required voltage is kept in the power terminal to drive the circuit when the output is turned on. The power reset circuit 228 prohibits the output during the period from the time when power is supplied from an external source to the time when the constant voltage output is stabilized. The shorting protection circuit 229, with the output terminal 238 thereof connected directly to the power supply, detects that an excess current has flowed in the output transistor 235, and activates the power reset circuit 238, thereby prohibiting the output.
The assumption of the circuit configuration described above will further facilitate the understanding of the problem points encountered in the process of cost reduction. As described above, the detection distance of the proximity sensor depends to a large measure on the outer diameter of the coil 231, and therefore many models of proximity sensors having different outer diameters are required to be prepared. Also, with regard to the specification of power supply and output, preparation of many models is required including DC or AC, three-line type (FIG. 27) or two-line type (FIG. 28), NPN or PNP, normally open or normally closed, cord output or connector output, etc. to meet the requirements of the users of the proximity sensor. Further, in accordance with the environment in which the proximity sensor is used, various models are required to be prepared including the metal case or resin case, brass or stainless steel, or short body or long body.
Conventionally, products with many combinations of these specifications have been designed and produced each time. It has been common practice, therefore, for the makers to supply a vast number of types of products. Under these circumstances, additional demand which may come from many users for an increased detection distance, a reduced product cost or an improved quality for each commodity is becoming more and more difficult to meet on the part of the makers.
In response to a demand for an increased detection distance, for example, the oscillation circuit is required to be designed for each of the custom ICs 201, 221 having different power specifications. Further, the capacitance value of the resonance capacitor 232 for determining the oscillation circuit constants, the resistance value of the regulation circuit 233 and the capacitance value of the integration capacitor 234 for determining the integration time constant are required to be designed for each coil having a different outer diameter specification. The resultant new addition of several types of custom IC would increase the external IC parts by several tens of types. Furthermore, the detection characteristic of the proximity sensor is considerably affected by the material of the body case. Even in the case where the specification is the same for the shielded type with the outer diameter M18 having the rated detection distance of 7 mm, for example, the metal case and the resin case produce different detection characteristics. To regulate them to the same detection distance, therefore, the oscillation constant is unavoidably different. This simple fact will facilitate the understanding of the difficulty of reducing the production cost through the concentration of the parts while maintaining the great variety of product specifications.
These problems of the conventional configuration derived from the magnetic circuit and the electrical circuit described above have various inconvenient effects on the production process of the proximity sensor.
FIG. 31 is a diagram showing the steps of the conventional production process of a proximity sensor of shielded type (which can be used in the form buried in a mounting metal) having a metal body case.
In this drawing, the first step (A) is to produce a detection end assembly 304 by assembling the coil 301, the ferrite 302 and the parts package board 303 integrally. In the next step (B), the coil 301 and the parts package board 303 are soldered on the detection end assembly 304. In the next step (C), the detection end assembly 304 that has been soldered is subjected to an operation check test (distance inspection, etc.) and the external appearance inspection. In the next step (D), the coil case 305 is filled with epoxy resin. In the next step (E), the detection end assembly 304 completed just now is accommodated in position in the coil case 305 filled with epoxy resin. In the next step (F), the resin is hardened by being left to stand for a predetermined length of time at room temperature, for example, and a detection end assembly 307 with a coil case is thus completed. In the next step (G), the operation test to adjust the distance is conducted on the detection end assembly 307 with the coil case completed in the preceding step. This distance adjustment is carried out by conducting the operation test while the detection end assembly 307 having the coil case is mounted in a dummy case 307a, and the adjustment is carried out based on the test result by replacing the chip resistor or otherwise. In the next step (H), the cord 308 is soldered to the detection end assembly 307 with the coil case thereby to complete the detection end assembly 309 with the cord. In the next step (I), the detection end assembly 309 with the cord, the cylindrical metal body case 310 and the cord clamp 311 are assembled integrally thereby to produce a semi-finished proximity sensor 312. In the next step (J), resin is injected in the semi-finished proximity sensor 312 and hardened thereby to produce a completed proximity sensor product 313. In the next step (K), the withstanding voltage test and the characteristic test are conducted to produce a tested final product 314.
In the fabrication process described above, the detection distance is adjusted in step (G) with the proximity sensor inserted in the dummy case 307a having the same material and the same shape as the metal body case actually mounted. The difference between the actual and dummy cases in the material, the plating thickness and the position of the metal case or the coil often causes a difference between the detection distance determined by adjustment of the detection distance and the actual detection distance measured with a metal case mounted thereon. As a result, a nonconforming product deteriorates the yield and sometimes requires the repair work. In addition, to cope with the difference in the material among metal cases, different dummy cases must be prepared, often wastefully requiring an increased number of dummy cases and frequent setup jobs.
Also, in view of the fact that the IC is configured in the form including the oscillation circuit and the output circuit, difference output circuits generates jobs of setup change (due to different power supplies) inefficiently in spite of the fact that the oscillation circuit remains the same.
Also, in the step of adjusting the detection distance with the dummy case 307a, the adjust resistance value is read by applying a terminal with a variable resistor to the adjust resistance wiring of the substrate having the circuits including the oscillation circuit mounted thereon. Due to the device difference and the resistance difference between different terminal-device connections, however, a difference develops between the actually read resistance value and the actually required resistance value. To fill up this gap, a correction value is determined. Since the correction value varies due to the variations of parts, ICs and the assemblies (position difference between the coil and the core, position difference between the core and the case, etc.), however, the correction values must be changed inefficiently for each part lot and each product lot. In addition, the difference in output status generates frequent jobs of setup change for a deteriorated production efficiency.
In the detection circuit adjust step, an adjust resistance value reading equipment is introduced. Due to variations between equipments, however, the adjustment is inconveniently made impossible for an assembly of a different model than the originally intended model.
Further, in the detection distance adjust step, the adjust resistance value reading equipment is affected by noises from other devices. Since a correction value is determined taking the particular noises into account, the correction value is inefficiently required to be changed each time of equipment movement or equipment part change.
This invention has been achieved in view of the above-described problem points of the conventional proximity sensor, and the object thereof is to provide the aforementioned type of sensor which permits simple commodity design and production against a great variety of commodity specifications.